Coordinated Multipoint (CoMP) transmission/reception is used in 3rd Generation Partnership Project (3GPP) networks to improve coverage, cell throughput, and/or system efficiency. In general, a mobile device, referred to herein as a UE, may be able to receive signals from and send signals to multiple cell sites. Given such a multiple-input, multiple-output (MIMO) setup, performance may be improved if the cell sites coordinate signaling transmissions and/or scheduling decisions. For example, downlink (DL) CoMP may coordinate signaling transmissions from multiple cell sites to a UE to improve interference avoidance. In another example, uplink (UL) CoMP may coordinate scheduling from different cell sites to take advantage of the multiple signals received at the multiple cell sites from a single UE, and therefore to significantly improve the link performance.
With UL CoMP, a serving cell schedules the UL transmissions for all UEs reporting to that serving cell. When transmissions from one or more of the UEs are also received by one or more neighboring cells with sufficient quality, the serving cell may request that these neighboring cells operate as cooperator cells that cooperate with the serving cell. It will be appreciated that a single cooperator cell may receive multiple distinct requests from multiple different serving cells, where the potential cooperator cell must efficiently decide which serving cells to cooperate with. Preferably, these decisions should be made so as to maximize the benefits achieved with CoMP pair-wise (serving-cooperator) interactions, and to minimize the probability of error.
It will be appreciated that one of the problems associated with UL CoMP stems from the fact that a particular cell, e.g., a cooperator cell, receives signals from both its own UEs (e.g., locally scheduled UEs) as well as from the UEs reporting to other serving cells (e.g., desired UEs). Because the cooperator cell has no control over the resources granted to the desired UEs, the Demodulation Reference Signal (DMRS) sequences of the locally scheduled and desired UEs are not guaranteed to be orthogonal and are not aligned in frequency in any special or predictable way. As a result, the desired UEs' signals are highly likely to interfere with signals from the cooperator cell's locally scheduled UEs. Further, simple serving-cooperator pairs are not guaranteed because three or more UEs, e.g., one locally scheduled UE and two or more desired UEs, may be using the same resources. Thus, channel estimates derived for the locally scheduled UEs and the desired UEs based on the corresponding DMRSs may not be sufficiently accurate. These inaccuracies are problematic because accurate channel estimates are necessary to fully enable the performance of a receiver. Further, because of the potential for overlapping DMRSs, the reliability (quantified as the probability of a miss and/or a false alarm) of the conventional DMRS-based decision making solution for CoMP systems is compromised.
Thus, there remains a need for improved channel estimation techniques for UL CoMP systems that introduce an efficient and reliable detection solution.